I have always shot a .22 short or a subsonic or suppressed .22 from a distance. then slowly got closer and closer as someone else was with the dog.
then went to the shotgun from a distance and had another person with the dog and would throw a toy, shoot the shotgun and they would play fetch with the shotgun noise.
gradually get closer and closer to the point of being able to have the dog understand that the shotgun is no big deal no matter how close.

My dogs never cared at all about shotgun noise and went very quickly. My sisters dog was too old and never got it when I tried to take her hunting. she is strictly a home/family dog. poor breeding lines, she got her from a mall pet store. My dog and my dads dog are brothers, picked them up from a breeder in denver, great lines.

not sure about longmont. had my dog get some training way down south by elizabeth, other than that I just did everything myself.

find an open field that is public ground and use that. like pawnee. or knock on a farmers door and ask if you can work your dog in his CRP ground or something. I hardly think they would care.

My dog is dang near full on house dog until hunting season rolls around. I work with him on occasion to keep him up, just whenever I have time and the mood strikes me.